In the aforementioned earlier work and generally in the art of scrubbing a furnace exhaust gas with a washing liquid, it is known to use a differential-pressure washer which comprises at least one annular-gap washer. An annular-gap washer, as this term is generally used herein, is a duct through which the gas to be scrubbed is passed and provided with a central body which defines an annular gap or annular constriction with a wall of the duct. A pressure differential is generated across this constriction and accelerates the gas threthrough. Water or another washing liquid is sprayed into the gas upstream of the gap so that the water droplets intimately contact the gas as the mixture traverses the annular gap.
In gas cleaning for metallurgical furnaces, the blast furnace is connected at its top with a differential pressure washer comprising at least one gas-conducting duct in which the annular-gap washer is provided. The annular-gap washer comprises the annular-gap canal and the axially shiftable insert body defining the annular gap in the latter. The washing agent is, as noted, sprayed into the gas stream upstream of the insert body, with reference to the direction of gas flow, and this body can be, as described in the aforementioned patents, connected in a control circuit for regulating the pressure of the exhaust gas at the gas outlet of the blast furnace. A pressure sensor may respond to the pressure within the blast furnace and can be connected in a control circuit for the servomechanism displacing the insert body to increase or decrease the gap width as required.
For the purposes of the present description, the term "annular-gap passage" will be used to describe not the entire duct of the annular-gap washer but only that portion of the duct which is directly juxtaposed with a surface of the insert body to define a constriction therewith. In general the annular-gap washer has an inlet whose diameter corresponds to the diameter of the duct head of the annular gap and an outlet whose diameter can be less than that of the duct.
In conventional blast furnace gas cleaning installations using annular-gap washers of the aforedescribed type, the following phases of gas cleaning operation can be discerned:
a transfer of the particles in the dust-containing gas to the surface of the liquid droplets or film of the washing liquid;
a removal of the particles by their entrainment with the liquid; and
the collection of the liquid with the dust particles from the dust removal apparatus.
In general, these steps for the scrubbing of a gas do not differ in the first phase from other types of scrubbers. Removal of the dust particles by the transfer to the liquid, however, is characteristic of the annular-gap washer since effective dust removal takes place only with high input speeds of the gas. It appears that the usual annular-gap washer operates in part by the venturi principle. It utilizes the fact that in a venturi nozzle, a pressure differential across a constriction is converted into a velocity increase and hence extremely high velocities can be developed within the annular-gap.
The highest velocities develop at the throat of the classical venturi scrubber so that the liquid is dispersed. Gas velocities of 20 to 120 meters/per second and more can be attained and the overall dust removal can exceed 99%.
Accordingly the conventional annular-gap washer converges in the direction of gas flow and the surfaces of the insert body has a corresponding convergence so that the diameter of the annular gap decreases in the direction of gas flow although the width (radial dimension) of the gap may remain constant between the inlet and the outlet sides of the annular-gap washer. Since the dust particles present in the hot gas can act as nuclei for condensation, the gas volume traversing the annular-gap decreases as condensation proceeds between the inlet and outlet sides of the annular gap.
Another advantage of the conventional annular-gap washer is that it can be used to control the pressure in the head of the blast furnace. In other words the inserted body can function as an adjustment element for regulating the pressure of the exhaust gas at the head of the furnace with the aid of the aforementioned control circuit.
In gas cleaning apparatus for a high pressure blast furnace in which the exhaust gas is originally at a pressure of about 3 atmospheres gauge, it is generally desirable to reduce the pressure in the annular gap washer to 1.2 or 1.1 atmospheres gauge while processing large quantities of gas.
The problems with such systems have generally centered on corrosion and erosion of the apparatus resulting from the high velocities and high volumetric rates of flow of the gas and can only be solved, with limited success, by using special corrosion resisting materials.
In practice, moreover, it has been found that several annular-gap washing stages may be necessary for the desired degrees of pressure reduction in the annular-gap washer. Furthermore the pressure reduction may require a prewashing or prescrubbing step for coarse separation of the dust, an adjustable differential pressure washer, and a droplet separator with a clean-gas takeoff in succession along the duct leading from the blast furnace. The differential pressure washer may be the exclusive unit for controlling the pressure in the blast furnace by the control circuit connected to its shiftable insert body and the differential pressure washer itself may comprise two annular-gap washers, the first serving for the control of the pressure and the second being connected to an expansion turbine. The two annular-gap washers are disposed one behind the other and are provided with a bypass duct which is branched behind the annular-gap washer, in the direction of gas flow, to the pure gas takeoff. The bypass duct can be provided with a control valve and the expansion turbine. The arrangement improves the ability to control the pressure in the gas furnace and insures the desired level of pressure drop in the gas derived therefrom.